The invention relates to a memory arrangement with ordered writing, particularly intended for use with a radar image display unit. The arrangement comprises a memory, commonly referred to as a "main memory", for storing data words at respective addresses, an input circuit having an output connected to the data input of the main memory, and at least one input for receiving input words to be stored in the main memory at respective addresses.
Arrangements of this kind used in association with radar image display units make it possible to produce an image whose luminance does not depend on the radar scan. Data words which represent the amplitudes of radar echoes are written into the main memory at addresses which correspond to the positions of the echoes on the radar image. Writing is effected at a slow rate determined by the radar scan. To obtain a good image a cathode-ray tube of the type used in television is employed. The video information represents the data words written into the main memory at a fast rate, which is displayed by the scanning operation of the cathode-ray tube.
Often the center of the radar scan is shifted, and the data stored in the memory before the shift must be updated to present an accurate display to the observer.
An arrangement of the type described in the opening paragraph by means of which it is possible to effect this updating is known from French Pat. No. 2,459,486 which corresponds to United Kingdom Pat. No. 2,052,910. This prior art arrangement comprises an input circuit including a limiter connected to the output of the memory which functions to reduce the value of the word which represents the amplitude of the echo already stored, and a comparator provided to compare the values of the words at the output of the limiter and the word which is to be recorded; the word which is ultimately recorded is the word which has the highest value. Thus, the value of the words representing the echoes already stored decreases with time, which shows on the display screen by a gradual dimming.
This arrangement has the drawback that to provide this gradual dimming it is necessary for the words to be composed of a large number of binary elements. Thus the main memory must have a large capacity.